Well - I replaced some of the files and discovered that the eCart Definition file was the culprit and what was breaking my store. I replaced it with one from the original files I downloaded.
As to what you suggested:
"If you would like to add an additional field to the products you will first need to add this column to your db. Once you have done that you can update the product insert and update pages to include a form element for the new value. The next thing to do would be to update the DataAssist Insert and Update server behaviors so that the new value will be recorded in the db."
I've added and populated the field in my database. How do I update the product insert and update pages then? Using DataAssist that is. I'd like to do this in a way that doesn't break the store. I'm now afraid of just diving in and changing things!
Thanks for your help. My other (paypal) problem is still happening with my other store.(I have another post about that.)